If I remove the custom agent from the dump database stage and define the image at the top-level as docker-plus, it works. Now that I've added the custom agent at that stage (the one for Build CSS works fine) it doesn't work, both with agent any at the toplevel nor with the image docker-plus at the top level.
The exact error message is:
Couldn't connect to Docker daemon at http+docker://localhost - is it running?
If it's at a non-standard location, specify the URL with the DOCKER_HOST environment variable.